1. Field of the Invention
The present invention relates to systems for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. Such a system is called a variable frequency drive, or VFD.
2. Description of Related Art
Certain types of VFDs, particularly medium voltage drives, utilize an integral isolation transformer to improve the harmonics performance, and also, in some cases, to provide multiple floating secondary windings so that the inverter cells on the separate windings can be connected together in certain configurations to increase the output voltage level. Besides the transformer, the drives also typically include high power switching mechanisms, control power circuitry, and in some cases softcharge circuitry on the primary of the transformer.
U.S. Pat. No. 5,625,545 discloses a pulse-width-modulation drive for controlling medium-voltage alternating current motors. U.S. Pat. No. 6,014,323 discloses a multiphase power converter. Each one of these patents is incorporated by reference in its entirety.
All existing VFDs intended for installation in outdoor environments utilize a dry-type transformer, which is not suitable for outdoor environments, and also use components that can only be used at high voltage level inputs at high cost and with a large footprint.
Although there are many designs for VFDs that are well known in the art, considerable shortcomings remain. What is needed is a VFD that can be completely sealed for outdoor environments, and that can accept high input voltage.